On migration and the birds that were frozen on the way (and never made it)

"Migration is an instinctive phenomenon, which produce some neurophysiological mechanisms acquired by a long process of natural selection, transmitting hereditary. These mechanisms are stimulated by external factors, mainly by the photoperiod (relation between the number of daytime and night-time hours) causing a stimulation of the gland pituitary that secretes hormones (gonadotrophins) who along with other Act on the metabolism of fats and the molting process. This process is determined for each species in each geographic location."